I actually have several types of air purifiers but I think that the main things to look for are that it is rated for at least twice the space of the room that it is in, that it produce the least amount of ozone that you can find( they all produce some, even the ozone free ones),and that it has a HEPA filter that traps as small of a particle size as you can find.

We just purchased the RABBIT AIR MINUS A2. Five filters/heap/charcoal activated/ medium filter/pre filter/ and a customized filter for your needs. Has an auto mode or manual, Just go to Rabbitair.com. Also emits almost 0 ozone, one of the lowest out there. Works great!!